This invention pertains to a process and catalyst for the hydro-oxidation of olefins, such as propylene, by oxygen in the presence of hydrogen to olefin oxides, such as propylene oxide.
Olefin oxides, such as propylene oxide, are used to alkoxylate alcohols to form polyether polyols, which find significant utility in the manufacture of polyurethanes and synthetic elastomers. Olefin oxides are also important intermediates in the manufacture of alkylene glycols, such as propylene glycol, and alkanolamines, such as isopropanolamine, which are useful as solvents and surfactants.
Propylene oxide is produced commercially via the well-known chlorohydrin process wherein propylene is reacted with an aqueous solution of chlorine to produce a mixture of propylene chlorohydrins. The chlorohydrins are dehydrochlorinated with an excess of alkali to produce propylene oxide. This process suffers from the production of a low concentration salt stream. (See K. Weissermel and H. J. Arpe, Industrial Organic Chemistry, 2.sup.nd ed., VCH Publishers, Inc., New York, N.Y., 1993, pp. 264-265.)
Another well-known route to olefin oxides relies on the transfer of an oxygen atom from an organic hydroperoxide or peroxycarboxylic acid to an olefin. In the first step of this oxidation route, a peroxide generator, such as isobutane, ethylbenzene, or acetaldehyde, is autoxidized with oxygen to form a peroxy compound, such as t-butyl hydroperoxide, ethylbenzene hydroperoxide, or peracetic acid. The peroxide is used to epoxidize the olefin, typically in the presence of a transition metal catalyst, including titanium, vanadium, molybdenum, and other metal compounds or complexes. Along with the olefin oxide produced, this process disadvantageously produces equimolar amounts of a coproduct, for example, an alcohol, such as t-butanol or methylphenylcarbinol, or an acid, such as acetic acid. Coproducts such as t-butanol and acetic acid must be recycled or their value must be captured in the market place. Other coproducts must be further processed into products of commercial value; for example, methylphenylcarbinol must be dehydrated to form styrene. (Industrial Organic Chemistry, ibid., pp. 265-269.)
More recently, the direct oxidation of olefins, such as propylene, with oxygen in the presence of hydrogen and a catalyst has been reported to yield olefin oxides, such as propylene oxide, as illustrated in EP-Al -0,709,360. It is taught that the catalyst comprises ultrafine particles of metallic gold deposited on titanium dioxide, preferably, the crystalline anatase phase. This catalyst exhibits a disadvantageously short lifetime. Moreover, when operated at temperatures of greater than about 100.degree. C., the catalyst exhibits low olefin oxide selectivity and high water production.
Other hydro-oxidation processes are known, for example, as described in international patent publications WO 98/00413 and WO 98/00415, wherein an olefin, such as propylene, is reacted with oxygen in the presence of hydrogen and a catalyst comprising gold deposited on a titanosilicate support or gold deposited on a support comprising a disorganized phase of titanium dispersed on silica. International patent publication WO 98/00414 describes a similar process wherein the catalyst comprises gold and a promoter metal, such as a Group 1, Group 2, or lanthanide rare earth metal, deposited on a titanium-containing support. The catalysts of these references achieve better lifetime and better hydrogen efficiency at comparable olefin selectivity when compared with the catalyst of EP-A1-0,709,360. The catalyst of WO 98/00414 also exhibits higher activity than the catalyst of EP-A1-0,709,360. Nevertheless, improvements in activity, lifetime, and hydrogen efficiency are still desirable.